The present invention relates to a silicon carbide-based porous body used in a filter for purification of automobile exhaust gas, a catalyst carrier, or the like, as well as to a process for production of such a silicon carbide-based porous body.
Porous honeycomb structures are in wide use as a filter for capturing and removing the particulate substance present in a dust-containing fluid (e.g. exhaust gas emitted from diesel engine), or as a catalyst carrier for loading thereon a catalyst component capable of purifying the harmful substances present in an exhaust gas. It is known that as a material constituting such a honeycomb structure, there are used refractory particles such as silicon carbide (SiC) particles and the like.
As a specific technique related thereto, there is disclosed, in, for example, JP-A-6-182228, a porous, silicon carbide-based catalyst carrier of honeycomb structure, obtained by using, as a starting material, a silicon carbide powder having a given specific surface area and a given impurity content, molding the material into a desired shape, drying the molded material, and firing the resulting material in a temperature range of 1,600 to 2,200xc2x0 C.
Meanwhile, there are disclosed, in JP-A-61-26550, a process for producing a vitrifying material-containing refractory product, which comprises adding a vitrifying material to an easily oxidizable material or a refractory composition containing an easily oxidizable material, mixing, kneading and molding them together with a binder, and open-firing the molded material in a furnace containing a non-oxidative atmosphere; and, in JP-A-8-165171, a silicon carbide molded material obtained by adding, to a silicon carbide powder, an organic binder and inorganic binders of clay mineral series, glass series and lithium silicate series and molding the resulting material.
Also, in JP-A-6-182228 is introduced a process for producing a conventional porous, silicon carbide-based sintered body, which comprises adding, to silicon carbide particles as an aggregate, a binder such as vitreous flux, clayey material or the like, molding them, and firing the molded material at a temperature at which the binder melts.
Further, as to a high-temperature use ceramic filter produced by molding refractory particles which consist of silica sand, a ground pottery, a metal oxide (e.g. Al2O3, TiO2 or ZrO2), silicon carbide, nitride, boride, other refractory material, or the like and which are adjusted to a given grain size, to a porous, bottomed cylindrical material using a refractory binder such as water glass, frit, glaze or the like, there are disclosed, in JP-B-61-13845 and JP-B-61-13846, the preferred average particle diameter and particle size distribution of refractory particles, the preferred porosity, average pore diameter, pore volume and partition wall thickness of cylindrical material, etc. In the sintering (necking between particles) caused by the recrystallization of silicon carbide powder per se, shown in JP-A-6-182228, the silicon carbide component vaporizes from the surfaces of silicon carbide particles and the vaporized silicon carbide component condenses at the contact areas (necks) between silicon carbide particles; as a result, the necks grow and the particles are bonded to each other. There are problems, however, that this method brings a high cost since a very high firing temperature is required to be employed in order to vaporize silicon carbide, and that the yield after firing is reduced since a material of high thermal expansion coefficient is required to be fired at a high temperature as well.
Further in JP-A-2000-218165 are disclosed a honeycomb filter obtained by forming a silica film for increased strength, on the inner walls of the pores of a porous silicon carbide sintered body, and a process for production of such a honeycomb filter. This honeycomb filter has a sufficient mechanical strength but has yet undissolved problems in the production cost, thermal conductivity, etc.; therefore, the fluid thereof is desired.
Meanwhile, the technique of bonding a silicon carbide powder (as a raw material) with a vitreous material, shown in JP-A-61-26550 and JP-A-6-182228 uses a low firing temperature of 1,000 to 1,400xc2x0 C.; however, when the sintered body produced by the technique is used, for example, as a diesel particulate filter (DPF) for removing the particulates contained in the exhaust gas emitted from a diesel engine and the particulates collected by and deposited on the filter are burnt for reactivation of the filter, there occurs local heat generation caused by the low thermal conductivity of the filter, which has incurred the destruction of the filter. Further, the filter shown in JP-B-61-13845 and JP-B-61-13846 is porous but is a bottomed cylindrical material having a large partition wall thickness of 5 to 20 mm; therefore, there is a problem that the filter is not usable under the high space velocity (SV) condition like as a filter for purification of automobile exhaust gas.
In order to solve the above problems, the present inventors propose, in Japanese Patent Application No. 2000-113513, a porous honeycomb structure containing refractory particles, in particular, silicon carbide as an aggregate and metallic silicon, and a process for production thereof. In the patent application, a honeycomb structure is proposed which can be produced at a relatively low firing temperature at a low cost and which has a high thermal conductivity, a sufficient porosity and a high specific surface area.
The honeycomb structure, however, may have problems depending upon the special environment in which it is used and the manner in which it is treated. For example, it is known that silicon carbide, when heated under a low oxygen partial pressure, gives rise to oxidative decomposition according to the following formula (1), resulting in reduced strength and oxidation resistance. It is also known that metallic silicon, when heated under a low oxygen content atmosphere, vaporizes or, as shown in the following formula (2), generates a SiO vapor. It is further known that these Si and SiO of gaseous state cause violent heat generation when they are oxidized or carbonized.
SiC+O2xe2x86x92 SiO↑+CO↑xe2x80x83xe2x80x83(1) 
xe2x80x83Si+{fraction (1/20)}2xe2x86x92 SiO↑xe2x80x83xe2x80x83(2)
When a filter is burned for reactivation, oxygen is consumed; resultantly, the filter is exposed to a reducing atmosphere. Therefore, when a silicon carbide-based honeycomb filter having a structure bonded by metallic silicon is used as a DPF and then is reactivated, oxidation reactions under a low oxygen partial pressure, such as shown by the above formulas (1) and (2) may take place; and there has been a fear of, for example, the destruction of the filter caused by sharp temperature increase due to the oxidation of, in particular, metallic silicon.
Metallic silicon further has a property of easily dissolving in an acid when having no oxide film thereon. As a result, when a sintered body containing metallic silicon as a constituent is used as a DPF, the sintered body is exposed to an acidic gas atmosphere generated by the combustion of sulfur, etc. present in the fuel used; and there has been a fear, for example, the destruction of the filter caused by dissolution of metallic silicon.
In view of such a situation, the present invention aims at providing a silicon carbide-based porous body which contains refractory particles such as silicon carbide particles or the like and yet can be produced at a relatively low firing temperature at a low cost and which has a high thermal conductivity and is improved in oxidation resistance, acid resistance, chemical resistance to ash and particulates, and thermal shock resistance; a honeycomb structure which can be suitably used, for example, as a filter for purification of automobile exhaust gas by a treatment such as clogging of through-channels at its inlet or outlet, or as a catalyst carrier, even under a high SV condition; and a process for producing such a honeycomb structure.
According to the present invention, there is provided a silicon carbide-based porous body, characterized in that said body is a porous one which contains silicon carbide particles as an aggregate and metallic silicon, and has an oxygen-containing phase at the surfaces of silicon carbide particles and/or metallic silicon or in the vicinity of the surfaces thereof.
In the present invention, the oxygen content is preferably 0.03 to 15% by weight; the oxygen-containing phase is preferably amorphous and/or crystalline SiO2 or SiO; and the following relation is satisfied:
A/Bxe2x89xa71.3 
when the strength is taken as A (MPa) and the Young""s modulus is taken as B (GPa).
According to the present invention, there is also provided a honeycomb structure constituted by any of the above silicon carbide-based porous bodies.
Meanwhile, according to the present invention, there is provided a process for producing a silicon carbide-based porous body, characterized by adding metallic silicon and an organic binder to raw material silicon carbide particles, mixing them, molding the mixture to a predetermined shape, calcinating the molded material in an oxygen-containing atmosphere to remove the organic binder in the molded material, and firing the calcinated material to obtain a silicon carbide-based porous body wherein an oxygen-containing phase is formed at the surfaces of the silicon carbide particles and/or the metallic silicon or in the vicinity of the surfaces thereof.
According to the present invention, there is also provided a process for producing a silicon carbide-based porous body, characterized by adding metallic silicon and an organic binder to raw material silicon carbide particles, mixing them, molding the mixture to a predetermined shape, calcinating the molded material to remove the organic binder in the molded material, firing the calcinated material, and subjecting the fired material to a heat treatment in an oxygen-containing atmosphere to obtain a silicon carbide-based porous body wherein an oxygen-containing phase is formed at the surfaces of the silicon carbide particles and/or the metallic silicon or in the vicinity of the surfaces thereof.
In the above present invention, the heat treatment is carried out preferably in a temperature range of 500 to 1,400xc2x0 C.
According to the present invention, there is further provided a process for producing a silicon carbide-based porous body, characterized by adding metallic silicon and an organic binder to raw material silicon carbide particles, mixing them, molding the mixture to a predetermined shape, calcinating the molded material to remove the organic binder in the molded material, firing the calcinated material, then coating the surfaces of the silicon carbide particles and the metallic silicon with a fluid containing silicon and oxygen, thereafter subjecting the resulting material to a heat treatment to obtain a silicon carbide-based porous body wherein an oxygen-containing phase is formed at the surfaces of the silicon carbide particles and/or the metallic silicon or in the vicinity of the surfaces thereof. In the aforementioned present invention, the heat treatment is carried out preferably in a temperature range of 50 to 1,400xc2x0 C.
In each of the above-mentioned present processes for producing a silicon carbide-based porous body, the firing is carried out preferably in a temperature range of 1,410 to 1,600xc2x0 C.